Electrode lead arrangement for three phase electric furnace



H. ERNST Oct. 13, 1959 ELECTRODE LEAD ARRANGEMENT FOR THREE PHASEELECTRIC FURNACE Filed June 6, 1958 IN VENTOR. Z2

rnst V uftcarzzey United States Patent ELECTRODE LEAD ARRANGEMENT FORPHASE ELECTRIC FURNACE 2,908,736 Patented Oct. 13, 1959 contact elementswhich are not shown but are well known in the art.

Hans Ernst, Duisburg, Germany, assignor to DEMAG- ElektrometallurgieG.m.b.H., Duisburg, corporation of Germany Application June 6, 1958,Serial No. 740,443

Claims priority, application Germany June 8, 1957 4 Claims. (Cl. 13-9)Germany, a

This invention relates to electric arc furnaces and more particularly tothe electrode leads of a three phase electric arc furnace.

section.

Electrode leads in a three phase electric arc furnace A i are generallydisposed in planar spaced apart relation with one electrode lead midwaybetween the other two. In the operation of such furnaces, particularlythose where the current exceeds 20,000 amperes, it has been 5 i lexinductance which is a combination of self inductance found that whenequal currents are flowing in the outer electrodes, the arc voltages ofthese electrodes are not equal. Thls phenomenon was due to difierencesin the inductances of the electrode leads of prior art electric arefurnaces and often resulted in unequal power output in the two outerelectrodes with consequent nonuniform electrode consumption and heatradiation.

Prior art arrangements for minimizing this unbalance in the inductancesof the phases, for example inserting a balancing transformer between thetwo outer phases, were expensive and often inefficient.

It is a primary object of the invention to minimize the inequality ofinductances between the electrode leads in a three phase electric arcfurnace.

Other objects and advantages of the invention will be apparent from thefollowing detailed description of the invention taken in view of theaccompanying drawing in which:

Fig. 1 is a vertical sectional view through an electric arc furnacehaving electrode leads according to a preferred embodiment of theinvention;

Fig. 2 is a sectional view through the electrode leads taken along line2-2 of Fig. 1;

Fig. 3 is a modification of the electrode leads shown in Fig. 2; and

Fig. 4 is a sectional view of the electrode leads taken along line 4-4of Fig. 1. I

Referring to the drawing, Fig. 1 shows a three phase electric furnace 10having a shell 12 with suitable refractory lining and three electrodes,only two of which 14 and 16 are shown, supported on and secured toelectrode supporting arms 18 by suitable electrode clamping devices 20which are well known in the art. Each of the electrode supporting arms18 is mechanically connected to a suitable positioning device such as ahy draulic ram 22 which is controlled by a servomechanism indicated at24 and which, in turn, responds to signals derived from the arc currentand are voltage by means that are not shown but are well known in theart.

The three electrodes are electrically connected to the phases of a threephase transformer 26 by electrode leads 28. Each of the electrode leads28 includes a first bus bar 30 secured to a transformer terminal 32, asecond bus bar 34 mounted on its respective electrode supporting arm 18by support members 36, and a flexible center portion 38 electricallyconnecting first bus bar 30 to second bus bar 34 and which allows theelectrodes to be raised and lowered without stressing bus bars 30 and34. Conducting members 40 electrically connect second bus bars 34 totheir respective electrodes through Throughout the descriptionand theappended claims, the expression mean geometric distance of the crosssection is intended to be generic to circular as well as noncircularcross sections such as shown in Fig. 2 and to bunched conductors asshown in Fig. 4 and to be the equivalent of the mean geometric radiuswhich is a mathematical quantity commonly used in electrical engineeringin the determination of the total flux linkages in a conductor ofcircular cross section. The inductance per unit length of each of theelectrode leads is a comand mutual inductance, and the inductances perunit length of the three electrode leads in a three phase furnaceenergized by balanced three phase currents having a sequence ABC aregiven by the expressions:

a at ac' bc E 5% L K(ln gla r r and r are the mean geometric distancesof the cross sections of the leads of the phases A, B, and C,respectively.

When the electrode leads 28 are disposed in a common plane and thedistances d and d are equal as shown in Fig. 2, the expressions for theinductance per unit length of the electrode leads becomes:

It can be seen from these expressions that the inductances of theelectrode leads of phases B and C can be made equal either by varyingthe distance between leads or the mean geometric distances of the crosssections of the leads. However, because the inductance increases as thedistance between electrode leads increases and because it is desirableto keep this inductance as small as possible, the electrode leads arepreferably disposed as close together as electrical clearance willpermit. In accordance with the invention, the inductances of theelectrode leads of phases B and C are equalized by metric distances oftheir cross sections. This desired relationship in the sizes of theleads is determined by equating the inductances L and L of these leads:

From this expression the relative dimensions of the electrode leads ofphases-B and C to achieve equality of inductance can be readilydetermined. This relationship is maintained for all three pontions ofthe electrode leads, i.e., the first bus bars 30, the second bus bars34, and the flexible centerportions 38.

Each flexible conducting portion 38 generally comprises a group ofbunched individual conducting members X, Y and Z as shown in Fig. 4. Theterm mean geometric distance of the cross section is not conventionallyapplied to such bunched conductors, but in an analogous manner theinductance per unit length of such a bunched conductor is dependent uponboth the mean geometric distance of the cross section of the individualconductors and the distances of the individual conductors from eachother. In accordance with the invention the individual conductors X, Yand Z of the electrode lead of phase C are either spaced from each othera greater distance than that between the individual conductors X, Y andZ' of the electrode lead of phase B or have a different size than theindividual conductors of phase B, or a combination of both such factorsmay be utilized as illustrated in Fig. 4. This variation in size andspacing between individual conductors to effect equal inductances of thetwo outer flexible portions most.

effective when equal numbers of individual conductors are utilized inthe two leads and the conductors have different spacing in a verticalplane as shown in Fig. 4.

While only a preferred embodiment of the invention has been illustratedand described, many modifications and variations thereof will be obviousto those skilled in the art, and consequently it is intended to cover inthe appended claims all such variations and modifications which fallwithin the true spirit and scope of. the invent-ion.

I claim: l a

1. In combination with a three phase electric arc furnace having threeelectrodes, an electrode lead connecting each of said electrodes with asource of three phase electrical energy, said electrode leads beingspaced relative to each other, one of said electrode leads beingdisposed substantially midway between the other two, said 2. In acombination with a three phase electric arc furnace having threeelectrodes energized by a three phase transformer, an electrode leadconnecting each of said electrodes with one of the phases of saidtransformer, said electrode leads being spaced relative to each other,one of said electrode leads being disposed substantially midway betweenthe other two, said electrode leads being substantially coplanar througha substantial portion of their lengths, the logarithm of the meangeometric distance of the cross section of one of said other electrodeleads being equal to the square root of the sum of the squares of thelogarithm of the mean geometric distance of the cross section of thesecond of said other electrode leads plus a constant.

3. In combination with a three phase electric arc furnace having threeelectrodes, a three phase transformer, three electrode leads, one ofsaid electrode leads connecting each of said electrodes with one of thephases of said transformer, said electrode leads being spaced relativeto each other with one of said electrode leads disposed substantiallymidway between the other two, each of said electrode leads including afirst bus bar a portion electrically connected to its correspondingelectrode, a second bus bar portion connected to one of the phases ofsaid transformer, and a flexible conducting portion connecting one ofsaid first bus bars with the corresponding one of said second bus bars,correspond-- r ing bus bar portions being substantially coplanar, saidflexible portions of said electrode leads being of substantially equallength, .the logarithm of the mean geometric distance of the crosssection of each of the portions of one of the outer electrodeleads'being equal to the square root of the sum of the squares of thelogarithm of the mean geometric distance of the cross section of thecorresponding portions of the other of said outer electrode leads plus aconstant.

4. In a three phase electric arc furnace having three electrodes, anelectrode lead connecting eachof said electrodes with one of the phasesof a three phase source of electrical energy, said electrode leads'being spaced from each other with one of said electrode leadssubstantially midway between the other two, each of said leads includinga first bus bar portion connected to one of said electrodes, a secondbus bar portion connected to said source of electrical energy, and aflexible portion connecting said first and second bus bar portions, the

corresponding bus bar portions being substantially coplanar, the meangeometric distance of the cross section of each of said portions of oneof the outer electrode leads being greater than the mean geometricdistance of the cross section of the corresponding portion of the otherof said outer leads, the flexible portion of each electrode leadincluding a group of individual conductors, the individual conductors ofsaid one electrode lead having a different effective distance from eachother than the individual conductors of the other outer electrode lead.

No references cited.

